1. Field of the Invention
The present invention relates to a method and an apparatus which, in a non-destructive inspection method for evaluating rear surface flaws, embedded flaws, or material characteristics using an electromagnetic technique, increase an inspection sensitivity to the rear surface flaws, the embedded flaws, or the material characteristics by locally heating a front surface of an inspection subject, thereby increasing currents supplied on a rear surface side of the inspection subject. The present invention particularly relates to a preferred technique for carrying out a non-destructive inspection with a high sensitivity to rear surface flaws, embedded flaws, or material characteristics when an inspection subject is a thick member.
2. Description of the Related Art
With a background that flaw acceptance criteria and damage tolerance designs are correspondingly applied to structures such as a nuclear plant, there is required a non-destructive inspection method providing a high flaw detection capability and evaluating shapes of flaws. On the other hand, these structures are often constructed using thick members, and it is difficult to detect rear surface flaws and embedded flaws of these thick members using generally available non-destructive inspection methods using electromagnetic techniques.
Conventionally, as one of the non-destructive inspection methods using the electromagnetic techniques, there is known a potential difference method which non-destructively inspects flaws or material characteristics by supplying currents to a conductive inspection subject, and then measuring an electric potential difference on a front surface of the inspection subject. The potential difference method includes methods using an AC source and a DC source as a current source thereof, which are respectively referred to as the AC potential difference method, and the DC potential difference method. If an AC with a high frequency is used, most currents flow along a front surface (skin effect), and the AC potential difference method is thus not suitable for flaw detection on a rear surface side of an inspection subject. On the other hand, in the DC potential difference method, DC currents flow through an inside as well as along a front surface of an inspection subject, and a current field is thus disturbed by embedded flaws and rear surface flaws in addition to front surface flaws. Consequently, a non-destructive inspection can be carried out for the above flaws by measuring the disturbances as changes in the electric potential difference on the front surface of the inspection subject. However, if an inspection subject is a thick member, only weak DC currents are supplied on the rear surface side of the inspection subject, and there is such a problem that a detection sensitivity to flaws on the rear surface remarkably decreases.
On the other hand, an eddy current flaw detection method is known as a non-destructive inspection method using the electromagnetic technique. According to the eddy current flaw detection method, a coil to which an AC is supplied is placed in contact with or with a narrow gap to a top of a conductive inspection subject, and eddy currents are induced on the inspection subject due to an electromagnetic induction phenomenon. If a flaw is present in the inspection subject, changes in the eddy currents caused by the flaw are detected as a change in the impedance of the coil. Though the eddy current flaw detection method presents an excellent sensitivity to front surface flaws in principle, the skin effect attenuates the eddy currents in the thickness direction, and thus the method is not suitable for flaw detection on a rear surface side of an inspection subject, and the application thereof is conventionally limited to front surface flaw detection, and thin plate flaw detection.
In addition, the electric potential difference on the front surface of the inspection subject and the impedance of the coil measured by the above-described electromagnetic techniques correlate to physical properties of a material such as an electric conductivity, a magnetic permeability, and a film thickness, these physical properties of the material in a range as far as the currents flow are thus non-destructively inspected using the electromagnetic techniques. However, if an inspection subject is a thick member, currents supplied on a rear surface side are weak, and it is thus extremely difficult to detect changes in a material on the rear surface.
As described above, according to the conventional electromagnetic techniques, only weak currents are supplied on a rear surface side of an inspection subject, and it is thus difficult to detect rear surface flaws, embedded flaws, and material properties of a thick member.